Thirty-one other people have previously pleaded guilty to their roles in the operation, the attorney's office said.

U.S. Attorney Erica MacDonald said the defendants preyed on foreign-born women from Thailand. They were shipped to cities around the United States, including Minneapolis, where they were sold to "sex buyers." During a press conference at the federal courthouse in St. Paul, authorities called this one of the largest "modern-day slavery" cases ever.

U.S. Attorney Erica MacDonald and other officials announce five convictions in connection with an international sex trafficking operation.

KSTP/Beth McDonough

One of the "houses of prostitution" where the trafficked women were forced to engage in sex with strangers for up to 12 hours a day, according to the U.S. Attorney's Office.

The women were "sold like cattle until they paid off a massive bondage debt," MacDonald said.

"These are people's mothers. These are people's daughters. These are people's sisters. These are people's aunts," she added.

"It started in Minnesota with a tip in 2014 that a female was flying in from Phoenix for work. Our case agent worked the lead and we never gave up," Laura Provinzino, the prosecutor who handles major crimes, said.

The investigation involved multiple agencies, including the Department of Homeland Security and the Internal Revenue Service.

IRS supervisory special agent Todd Strom said the operation involved "tens of millions of dollars." A release from the U.S. Attorney's Office said the organization engaged in "rampant and sophisticated money laundering" to mask their illegal profits.

Investigators have so far recovered $1.5 million in cash and $15 million in money judgments secured through plea agreements, the release said.

Panida Rzonca works with the Thai Community Development Center in Los Angeles, which supports many of the victims, offering medical and mental care, housing and everyday needs.

"This is not the end for our victims," she said. "I hope that more victims come forward even though this verdict has been issued."

Senior litigation counsel Melinda Williams said there were "hundreds if not thousands of victims in this case, some of whom testified at trial."

"A very dedicated law enforcement team is looking for those women," she said.

Most of the victims were women in their 30s and 40s, officials said.

"They were coerced to participate in the criminal scheme through misleading promises of a better life in the United State and the ability to provide money to their families in Thailand," the attorney's office said.

According to the attorney's office, the women were sent to "houses of prostitution" when they reached the U.S., seven of them were located in Minnesota. They were "forced to have sex with strangers - every day - for up to 12 hours a day, at times having sex with ten men a day."

The money made was turned over to bosses with the criminal organization.

Prosecutors believe the guilty verdicts send a message to other victims.

"You're not alone, we've got your back and we will hold the offenders accountable," MacDonald said.

Provinzino said sentencing will take place next year.

"This is complete validation for the victims who endured the trafficking and the countless isolation they suffered, being shipped house to house and the threats made against them and their families," she said.